1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and first and second metal separators in a stacking direction. The membrane electrode assembly includes a first electrode, a second electrode, and an electrolyte membrane interposed between the first electrode and the second electrode. The surface area of the second electrode is larger than the surface area of the first electrode. A reactant gas passage for at least one reactant gas extends through the fuel cell in the stacking direction.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between separators to form a unit of a power generation cell for generating electricity. In use, generally, a predetermined number of the power generation cells are stacked together to form a fuel cell stack.
In the power generation cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the power generation cell, a fuel gas flow field (reactant gas flow field) for allowing the fuel gas to flow along the anode and an oxygen-containing gas flow field (reactant gas flow field) for allowing the oxygen-containing gas to flow along the cathode are formed in the surfaces of the separators. Further, a coolant flow field for allowing a coolant to flow in the direction parallel to the surfaces of the separators is formed in at least one of positions between the power generation cells.
As one type of the fuel cell, an internal manifold type fuel cell is known. In the internal manifold type fuel cell, reactant gas supply passages, reactant gas discharge passages, a coolant supply passage and a coolant discharge passage extend through outer regions of the power generation cells in the stacking direction. In the fuel cell, the reactant gas supply passage and the reactant gas discharge passage are connected by a connection region. However, the connection region intersects the seal line which is formed around the reactant gas supply passage and the reactant gas discharge passage. Therefore, for example, a reinforcement member needs to be provided along the seal line.
In this regard, Japanese Laid-Open Patent Publication No. 2002-83614 proposes a fuel cell in which it is possible to fully maintain the hermetically sealed state with a simple structure. As shown in FIG. 9, the fuel cell includes a unit cell 1 and separators 2, 3 sandwiching the unit cell 1. Another separator 4 is stacked on the separator 3.
A manifold 5 for supplying hydrogen extends through the unit cell in the stacking direction. The manifold 5 is connected to a groove 6 formed on one surface of the separator 3. The separator 3 has a through hole 7. The through hole 7 is connected to a fuel flow field 8 formed on the other surface of the separator 3. Seal members 9a, 9b are formed around the manifold 5 between the unit cell 1 and the separators 2, 3.
In the fuel cell, since the fuel flow field 8 is connected to the manifold 5, the groove 6 is formed on the surface opposite to the fuel flow field 8, and the groove 6 is connected to the fuel flow field 8 through the through hole 7. According to the disclosure, with a simple structure without any grooves which intersect the seal members 9a, 9b, the desired sealing performance can be achieved.
In the conventional technique, the separators 2 to 4 are carbon separators. Therefore, a flow field having a desired shape can be formed on both surfaces of the separators 2 to 4 individually. However, in the case of using a metal separator instead of the carbon separator, and the reactant gas flow field and the coolant flow field are formed on both surfaces of the metal separator, respectively, the shape of the reactant gas flow field formed on one surface of the metal separator limits the shape of the coolant flow field formed on the other surface of the metal separator. Further, since the metal separator is formed by press forming, in particular, for example, the shape at the end of the flow field cannot be designed freely.
Thus, if the conventional separators 2 to 4 are fabricated by using metal, it may not be possible to reliably provide the connection region connecting the through hole 7 and the fuel flow field 8. In this case, it is not possible to smoothly supply the fuel from the through hole 7 to the fuel flow field 8, and the power generation performance is deteriorated. Further, the unit cell 1 cannot be held by the metal separator, and the unit cell 1 may be damaged undesirably when a tightening load is applied to the unit cell 1.